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Global diversity of rotifers (Phylum Rotifera) in freshwater

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Rotifera is a Phylum of primary freshwater Metazoa containing two major groups: the heterogonic Monogononta and the exclusively parthenogenetic Bdelloidea. Monogononta contains 1,570 species-level taxa, of which a majority (1,488) are free-living fresh or inland water taxa. Bdelloidea contains 461 “species,” only one of which is marine, but with many limnoterrestrial representatives or animals of unknown ecology. Actual numbers may be much higher, considering the occurrence of cryptic speciation in Monogononta and the unsatisfactory nature of taxonomic knowledge. Rotifers, mostly monogononts, occur in all types of water bodies, worldwide. They are particularly diverse in the littoral zone of stagnant waterbodies with soft, slightly acidic water and under oligo- to mesotrophic conditions. The rotifer record is highest in the Northern hemisphere, which may be due to the concentration of studies in those regions. Diversity is highest in the (sub)tropics; hotspots are northeast North America, tropical South America, Southeast Asia, Australia, and Lake Baikal, endemicity is low in Africa (including Madagascar), Europe, the Indian subcontinent, and Antarctica. Although the lack of fossil evidence and of molecular phylogenetic studies are major hindrances, contrasting hypotheses on the origin and evolutionary history of Brachionus, Macrochaetus, and Trichocerca are presented.
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FRESHWATER ANIMAL DIVERSITY ASSESSMENT
Global diversity of rotifers (Rotifera) in freshwater
Hendrik Segers
ÓSpringer Science+Business Media B.V. 2007
Abstract Rotifera is a Phylum of primary freshwater
Metazoa containing two major groups: the heterogonic
Monogononta and the exclusively parthenogenetic
Bdelloidea. Monogononta contains 1,570 species-
level taxa, of which a majority (1,488) are free-living
fresh or inland water taxa. Bdelloidea contains 461
‘species,’’ only one of which is marine, but with many
limnoterrestrial representatives or animals of unknown
ecology. Actual numbers may be much higher,
considering the occurrence of cryptic speciation in
Monogononta and the unsatisfactory nature of taxo-
nomic knowledge. Rotifers, mostly monogononts,
occur in all types of water bodies, worldwide. They
are particularly diverse in the littoral zone of stagnant
waterbodies with soft, slightly acidic water and under
oligo- to mesotrophic conditions. The rotifer record is
highest in the Northern hemisphere, which may be due
to the concentration of studies in those regions.
Diversity is highest in the (sub)tropics; hotspots are
northeast North America, tropical South America,
Southeast Asia, Australia, and Lake Baikal, endemic-
ity is low in Africa (including Madagascar), Europe,
the Indian subcontinent, and Antarctica. Although the
lack of fossil evidence and of molecular phylogenetic
studies are major hindrances, contrasting hypotheses
on the origin and evolutionary history of Brachionus,
Macrochaetus, and Trichocerca are presented.
Keywords Monogononta Bdelloidea
Freshwater Biodiversity Zoogeography Review
Introduction
Rotifera (see Wallace et al., 2006 for a recent,
comprehensive introduction to the taxon) is a group
of primary freshwater invertebrates. Rotifers play a
pivotal role in many freshwater ecosystems. They are
ubiquitous, occurring in almost all types of freshwa-
ter habitat, from large permanent lakes to small
temporary puddles, and interstitial and capillary
water; from acidic mining lakes to natron lakes and
the open ocean, from hyperoligotropic Alpine lakes
to sewage ponds. They commonly occur in densities
up to 1,000 individuals per liter, and are important
filter-feeders on algae and bacteria. Their ubiquity
and abundance explain their standing as one of the
three main groups of freshwater zooplankton in
limnological studies, together with the ‘Cladocera’
(Anomopoda) and Copepoda, and as organisms used
in mass aquaculture. They are permanently and
obligatorily connected to aquatic habitats in all active
stages, only their resting stages are drought-resistant.
Guest editors: E. V. Balian, C. Le
´ve
ˆque, H. Segers &
K. Martens
Freshwater Animal Diversity Assessment
H. Segers (&)
Belgian Biodiversity Platform, Freshwater Laboratory,
Royal Belgian Institute of Natural Sciences, Vautierstraat
29, 1000 Brussels, Belgium
e-mail: Hendrik.Segers@naturalsciences.be
123
Hydrobiologia (2008) 595:49–59
DOI 10.1007/s10750-007-9003-7
Classically, three groups are recognized within the
Phylum Rotifera. The species-poorest is Seisonacea,
with only three species living epizootically on marine
crustaceans of the genus Nebalia. Most well-known
and diverse are the predominantly freshwater Bdel-
loidea and Monogononta. Molecular studies have
indicated that a fourth group, Acanthocephala, pre-
viously considered a separate Phylum of exclusively
endoparasitic organisms, actually belongs to Rotifera
(Mark Welch, 2000; Giribet et al., 2000). Little is
actually known about the phylogeny of rotifers, due
to a lack of modern comprehensive studies (but see
Sørensen & Giribet, 2006), and the lack of a robust
fossil record.
Rotifers are minute metazoans (50–2,000 lm),
characterized by the presence of an anterior ciliated
corona, a stiff body wall named lorica bearing variable
appendages, and a specialized pharyngeal organ, the
mastax, containing hard elements, termed trophi
(Fig. 1). Especially, the rotifer’s small size, capability
of phenotic plasticity and highly adaptable masticatory
apparatus are important elements explaining the suc-
cess of the group. Their propagules consist of single,
hard-shelled, and durable encapsulated cysts (monog-
ononts) or anhydrobiotic individuals (bdelloids).
These propagules being small and drought-resistant,
makes rotifers perfectly adapted to passive, aerial or
phoretic dispersal. Monogononts and bdelloids repro-
duce parthenogenetically. In monogononts, periods of
parthenogenetic reproduction are interspersed with
sexual phases (heterogony), but bdelloids are unique in
being the most diverse group of metazoans in which
reproduction is by diploid, mitotic parthenogenesis
only. The combination of their high dispersal capacity
and their parthenogenetic reproduction, enabling them
to establish or renew a population starting off from a
single resting stage, and to reach high effective
population sizes relatively quickly, makes them theo-
retically superbly apt (re)colonizers.
The ability of many bdelloids to shift from active to
anhydrobiotic stage enables them to live in particularly
ephemeral, even predominantly dry conditions such as
Fig 1 (a) Schematic
representation of a
Brachionus rotifer; (b)
Incudate trophi
(Asplanchna); (c)
Malleoramate trophi
(Sinantherina). Scale bars:
10 lm
50 Hydrobiologia (2008) 595:49–59
123
lichens or terrestrial mosses. As such, they should
probably be considered limnoterrestrial rather then
limnetic. Bdelloid rotifers, however, can at present
only be identified while alive and need to be examined
during feeding and creeping. Their study is, conse-
quently, tedious and very little if any information is
available on the ecology of the majority of them. So,
notwithstanding that the present project focuses on
limnetic representatives of these animals, I include
counts of the diversity and distribution of all freshwa-
ter bdelloid taxa, as it is not possible to distinguish
reliably between the two ecological groups.
Biodiversity of Rotifera
Data collection
Data on which the present analysis is based are restricted
to those rotifer taxa that are freshwater or brackishwater
and marine. Exclusively marine species have not been
included but are listed in the electronic appendix
(http://fada.biodiversity.be; see Fontaneto et al., 2006
for a recent review). The taxonomy follows recent tax-
onomic views as expressed in recent revisions of
selected rotifer families (Nogrady et al., 1995; Segers,
1995a,2003;DeSmet,1996; De Smet & Pourriot, 1997;
Nogrady & Segers, 2002), and numerous taxonomic
publications. When alternative taxonomies exist, a
splitting rather than lumping approach was followed.
Species that are insufficiently described and therefore
have to be considered species inquirenda are not
counted. A more complete account on the taxonomic
approach is provided in Segers (2007).
Distributional data are based on the literature review
of De Ridder (1986,1991,1994), De Ridder & Segers
(1997), Segers (1995b,2003) and recent articles (e.g.,
Jersabek, 2003; Ricci et al., 2003). Rare regional
records of species otherwise common in other regions
were critically assessed and eventually included only
after verifications of published illustrations or material.
The data are presented in Segers (2007) and in the
electronic appendix (http://fada.biodiversity.be).
Rotifer taxonomy and zoogeography: state
of the art
Before analyzing rotifer diversity and distribution, it
is necessary to give an account on the limitations of
the data. The usual caveat, that new species are still
to be discovered, applies, but there is more. Rotifer
taxonomy is almost exemplary of the taxonomic
impediment, as recognized by governments through
the Convention on Biological Diversity (see
http://www.biodiv.org/programmes/cross-cutting/
taxonomy/default.shtml). Serious knowledge gaps
exist in the taxonomic system of rotifers and trained
taxonomists and curators are (very) few. These defi-
ciencies have a significant impact on our ability to
understand the diversity and chorology of these ani-
mals. Rotifer taxonomy is all but adequate, an
observation that was already made some 25 years ago
(Dumont, 1980) but which still holds. Basic, detailed
morphological revisions still contribute significantly
to our understanding (e.g., Giri & Jose
´de Paggi,
2006). Molecular studies with an impact on taxon-
omy are still scarce. However, the work by Go
´mez
et al. (2002) on the economically important and
particularly well-studied B. plicatilis O.F. Mu
¨ller has
shown that the taxon, which was long treated as a
single but variable species, contains no less then nine
different, phylogenetically distinct lineages. Only few
of these are morphologically diagnosable (see Ciros-
Pe
´rez et al., 2001). Such cryptic speciation is proba-
bly common in rotifers, as hinted at by the
reproductive isolation of geographically separated,
yet morphologically identical strains of Asplanchna
brightwellii Gosse (see Snell, 1989). These problems
are further convoluted in bdelloid rotifers. Here, the
difficulties are not only the classic ones hampering
rotifer taxonomy (small size of the animals, scarcity
of useful morphologic features, high variability: see
Ruttner-Kolisko, 1989), but also the practical prob-
lem that, to date, only living and actively moving
animals can be identified or serve as a basis for tax-
onomic study. In addition, the animal’s unique
exclusively parthenogenetic reproduction implies that
most species concepts are inapplicable as theoretical
framework for their study. Clearly, the counts of
rotifer diversity as presented here are tentative and
should be interpreted with great caution.
Due to the caveat mentioned above, and because
identification of rotifers is difficult, rotifer literature is
littered with dubious records. Our knowledge on the
diversity and distribution of rotifers is moreover
biased by the uneven research intensity in different
regions (Dumont, 1983). There are only a few rotifer
families for which a large number of fairly reliable
Hydrobiologia (2008) 595:49–59 51
123
data is available. These are loricate taxa, which can
mostly be identified using external morphology of
contracted, fixed material, notably Brachionidae:
Pejler (1977) and Dumont (1983), Lecanidae: Segers
(1996), and Trichocercidae: Segers (2003).
Genus- and species-level diversity
A total of 1,570 Monogononta and 461 Bdelloidea
valid species are presently recognized worldwide
(Table 1). Of these, the vast majority (1488 monog-
ononts, 460 bdelloids) are either exclusively
freshwater or brackishwater and marine; only 70
described species are exclusively marine (Table 2).
The most diverse taxa are Notommatidae, with
Cephalodella as most speciose genus, the monogen-
eric Lecanidae, and Dicranophoridae. All of these
contain almost exclusively benthic-littoral or psam-
mon-inhabiting species, with a majority inhabiting
oligo- to mesotrophic, slightly acidic, soft waters.
The same holds for Lepadellidae; Brachionus is a
notable exception, as most of these prefer alkaline
and eutrophic conditions. These preferences are well
known and have been commented upon as early as
Harring & Myers (1928).
Beres et al. (2005) found that the distribution of
genera over families in rotifers is a hollow curve
distribution which fits a model given by Hubbell’s
unified neutral theory of biodiversity (Hubbell, 2001).
Basically, this distribution infers that there are
relatively numerous taxa containing only one or a
few subordinate taxa; that the relative frequency of
taxa decreases sharply with increasing number of
included subordinate taxa, whereas there are only a
few highly diverse taxa (e.g., Lecane: 200 species,
Cephalodella: 159 species). The same seems to hold
for the relation between genera and species in
Monogononta (Fig. 2), however, it is as yet unclear
what this may signify in respect to evolution or
biodiversity.
Rotifers, especially monogononts, form a rela-
tively diverse constituent of the fauna of stagnant
freshwater ecosystems. Dumont & Segers (1996)
calculated that a non-polluted lake with developed
weedy littoral would harbour about 150 species in
temperate, and up to 250 species in tropical regions.
This implies that 7.5–12.5% of all species globally,
and ca. one fifth of the regional fauna can be found in
a single locality. Myers’ (1942) intensive studies on
some lakes and ponds in and near the North-
American Pocono region (Pennsylvania) yielded
457 Monogononta and 32 Bdelloidea, which consti-
tute more than half of the known Nearctic rotifer
fauna in a relatively small region. This remarkably
high species diversity, which actually concerns
littoral and benthic rotifers, which are mostly present
in relatively low numbers, can be ascribed to fine
niche partitioning amongst rotifer species in combi-
nation with high micro- and macroscale habitat
heterogeneity, especially in littoral and benthic
environments. On the other hand, local diversity
can represent a sizable fraction of regional diversity.
This is probably a result of the high (re)colonization
and dispersal capacity of rotifers: available niches,
even if these are only temporarily present, are
relatively quickly filled by recruitment from resting
stages that may or may not already be present in the
habitat. This situation may be different from that in
pelagic habitats, where the presence of a large resting
propagule bank produced by locally adapted popula-
tions consisting of large numbers of individuals,
presents an effective barrier against newly invading
genotypes (the Monopolization Hypothesis: De Me-
ester et al., 2002). Alternatively, the observation may
be due to a lack of taxonomic resolution in littoral
rotifers.
Present distribution and main areas of endemicity
The most diverse and, not coincidently, best-studied
region is the Palaearctic, closely followed by the
Nearctic region (Map 1). A substantial research effort
resulting in a relatively high species record has been
devoted to the Neotropical region and, more recently,
the Oriental region. There are a fair number of
contributions on the Australian and Afrotropical
(Ethiopian) regions, but far less on Oceanic islands
(see Wallace et al., (2006) for a literature review).
That research intensity is largely responsible for this
ranking is best illustrated by the regional diversity of
taxonomically difficult illoricate taxa such as Dicr-
anophoridae and Notommatidae: the diversity of
these in the best studied Palaearctic and Nearctic
regions, where most rotifer taxonomists live(d), is
almost 7- to 8-fold that of the least studied African
region; this is much less so for the relatively easier
loricate taxa such as Brachionidae and Lecanidae.
52 Hydrobiologia (2008) 595:49–59
123
Antarctica is a special case; there are quite a few
studies but here rotifer diversity is markedly and
effectively lower then in other regions (Fig. 3).
Endemicity at higher taxonomic levels is rare in
rotifers. There is a single endemic free-living rotifer
family, the Nearctic (northeast North American)
Table 1 Number of genera per family, per region
Number of genera Palearctic Afrotropical Australian Oriental Nearctic Neotropical Antarctic Pacific Total
*
Monogononta
Asciaporrectidae 1 1 1
Asplanchnidae 3 2 2 3 3 3 1 3
Atrochidae 3 1 2 3 2 1 3
Birgeidae 1 1
Brachionidae 7 7 6 7 7 7 3 1 7
Collothecidae 2 1 2 2 2 2 1 1 2
Conochilidae 1 1 2 1 2 1 2
Dicranophoridae 14 5 8 5 12 6 2 5 19
Epiphanidae 5 4 5 5 5 3 2 4 5
Euchlanidae 4 4 4 5 4 4 1 2 5
Flosculariidae 7 6 7 6 7 7 1 4 7
Gastropodidae 2 2 2 2 2 2 2
Hexarthridae 1 1 1 1 1 1 1 1
Ituridae 1 1 1 1 1 1 1
Lecanidae 1 1 1 1 1 1 1 1 1
Lepadellidae 3 3 3 4 4 4 2 3 4
Lindiidae 1 1 1 1 1 1 1 1 1
Microcodidae 1 1 1 1 1 1 1 1
Mytilinidae 2 2 2 2 2 2 2 1 2
Notommatidae 15 9 11 9 15 10 3 5 18
Proalidae 4 3 3 2 4 3 3 4
Scaridiidae 1 1 1 1 1 1 1 1 1
Synchaetidae 3 3 3 3 4 3 2 4
Testudinellidae 2 2 2 3 2 2 1 3
Tetrasiphonidae 1 1 1 1 1 1 1
Trichocercidae 3 2 3 3 3 2 1 2 3
Trichotriidae 3 3 3 3 3 3 1 3
Trochosphaeridae 3 3 3 3 2 3 1 3
Subtotal: 94 70 80 78 94 76 23 40 108
Bdelloidea
Adinetidae 2 1 1 1 1 1 1 1 2
Habrotrochidae 3 3 3 1 3 2 1 2 3
Philodinavidae 3 1 2 1 2 2 3
Philodinidae 11 10 10 6 9 9 4 4 12
Subtotal: 19 15 16 9 15 15 6 7 20
Total: 113 85 96 87 109 91 29 47 128
*
Total number of species includes exclusively marine taxa, not included are Clariaidae (1 species, Claria segmentata Kutikova,
Markevich & Spiridonov, 1990), and 3 Seisonacea.
Hydrobiologia (2008) 595:49–59 53
123
Table 2 Number of species-level taxa per family, per biogegraphic region
PA NA NT AT OL AU PAC ANT End. Cosmo. World Mar.
Monogononta
a
Asciaporrectidae 3 2 0 00000 1 0 3
Asplanchnidae 11 11 10 9 12 9 2 0 2 8 15
Atrochidae 4 2 1 13200 0 1 4
Birgeidae 0 1 0 00000 1 0 1
Brachionidae 94 66 71 51 57 58 4 16 94 36 169 1
Brachionus 32 23 32 26 33 34 3 5 29 20 63
Keratella 21 22 18 15 12 15 0 5 26 7 48
Notholca 3113832206 27 2 401
Collothecidae 42 18 15 14 8 12 2 2 24 10 47
Conochilidae 5 7 5 55600 1 5 7
Dicranophoridae
b
137 93 21 19 15 24 5 6 98 9 181 39
Dicranophorus 36 38 10 12 8 8 1 0 21 7 52 1
Encentrum 6428324615 54 1 7831
Epiphanidae 16 10 10 99842 4 9 16
Euchlanidae 19 18 14 15 15 18 3 2 8 11 27
Flosculariidae 35 38 37 22 23 30 5 2 7 19 50
Gastropodidae 10 7 8 86700 2 6 12
Hexarthridae 11 11 7 84630 7 4 18
Ituridae 4 4 4 23500 0 2 6
Lecanidae 93 108 94 82 99 61 30 2 81 49 200
Lepadellidae 95 67 70 54 59 55 18 11 81 37 160 3
Lepadella 66 42 52 39 42 41 11 7 70 25 122 2
Lindiidae 7 11 4 23721 4 3 133
Microcodidae 1 1 1 1 1 1 1 0 1 1
Mytilinidae 21 10 14 12 12 12 1 2 13 8 29
Notommatidae 201 165 70 29 48 72 11 11 149 45 277
Cephalodella 118 79 37 6 26 31 14 8 93 16 159
Notommata 29 36 12 10 8 14 6 1 25 10 47
Proalidae 34 34 7 10 7 14 5 0 20 6 47 9
Scaridiidae 3 3 4 44311 3 2 7
Synchaetidae 38 26 18 13 15 17 3 0 16 12 45 12
Testudinellidae 19 19 19 18 15 17 1 0 19 9 40 1
Tetrasiphonidae 1 1 1 11100 0 1 1
Trichocercidae 50 53 45 39 41 43 18 4 13 34 70 2
Trichotriidae 13 11 15 12 11 10 1 0 10 9 23
Trochosphaeridae 13 8 1 13 10 13 0 0 5 9 19
Subtotal 980 805 566 453 486 511 119 63 663 345 1488 70
Bdelloidea
Adinetidae 17 8 6 7 5 12 1 6 7 5 20
Habrotrochidae 130 25 37 45 18 53 7 7 75 14 152
Habrotrocha 108 22 33 39 18 44 6 7 64 13 128
Philodinavidae 3 2 2 12200 3 0 6
Philodinidae 220 77 71 85 33 109 6 15 152 41 282 1
54 Hydrobiologia (2008) 595:49–59
123
Birgeidae. A number of endemic genera exist: In the
Palaearctic these are Pseudoharringia, the psammo-
biotic Wigrella, the European Alpine Glaciera and
the Baikalian Inflatana; in the Nearctic (northeast
North American) Rousseletia and the littoral
Streptognatha, and, probably, Pseudoploesoma (the
appurtenance of P. greeni Koste to this genus is
doubtful: De Smet & Segers, unpublished); in the
Oriental region Pseudoeuchlanis and Anchitestudinella;
and the Subantarctic (Kerguelen Island) Pourriotia.The
biogeographical relevance of these is, however, low:
all but Wigrella are monospecific, many (Glaciera,
Inflatana, Pseudoeuchlanis, Anchitestudinella and
Pourriotia) have only been found once. The fate of
Dorria is revealing: this monospecific genus was long
considered a rare northeast North American endemic
taxon, until it was found in southern Australia
and on Hawaii (Jersabek, 2003). More reliable, also
taxonomically, are Birgeidae, Streptognatha and
Pseudoploesoma; all three of these are northeast North
American. This concurs with the main center of
endemicity of Trichocercidae (Segers, 2003).
Endemic species occur in all regions and in all but
the species-poorest rotifer genera and families. The
count of endemics in Table 2, however, underrepre-
sents endemicity and complexity of the distributions
of rotifers: quite a few species technically occur in
more than one biogeographical region as accepted for
this study, yet are clearly restricted to a circumscribed
area (e.g., Keratella kostei Paggi occurs in Patagonia,
the Falkland Islands and South Georgia Island hence
both in the Neotropical and Antarctic region) or have
far more restricted ranges (e.g., the numerous Baika-
lian endemics, mostly of Notholca). Lecanidae is a
0
5
10
15
20
25
30
35
0 50 100 150 2 00
areneg fo rebmun
0
5
10
15
20
25
30
35
1
2ot 3
4
o
t 7
8to1
5
1o
t
63
1
t 23 o63
6o
t
4 127
8215
42
o
t
species per genus
areneg fo rebmun
a
b
Fig 2 Distribution of rotifer species diversity over different
genera. (a) normal representation, (b) number of species
(x-axis) sorted out in octaves
Table 2 continued
PA NA NT AT OL AU PAC ANT End. Cosmo. World Mar.
Macrotrachela 75 19 22 31 11 41 3 7 50 14 95
Mniobia 41 11 10 5 0 21 2 29 249
Philodina 35 17 14 24 6 18 1 5 28 10 50
Subtotal 370 112 116 138 58 176 14 28 237 60 460 1
Total 1,350 917 682 591 544 687 133 91 900 405 1948 71
PA: Palaearctic; NA: Nearctic; NT: Neotropical; AT: Afrotropical; OL: Oriental; AU: Australasian; PAC: Pacific Oceanic Islands;
ANT: Antarctic. End. = Endemics, Cosmo. = Cosmopolites, Mar. = Marine
a
Excluding Clariaidae, a monospecific family of exclusively parasitic animals living in terrestrial Oligochaeta
b
Excluding Albertia (4 species) and Balatro (7species), exclusively endoparasitic in Oligochaeta (both) and gastropods (Albertia);
Endemics: present in one region only
Cosmopolites: present in 5 or more regions
Marine: exclusively marine species
Hydrobiologia (2008) 595:49–59 55
123
good illustration of the diversity of distribution
patterns (Segers, 1996). Since this 1996 paper, over
30 Lecane have been added as valid, either as a result
of the application of a less inclusive taxonomic
concept or by the description of new species. In
general, ranges of Lecane have been refined and
counts of regional endemicity increased, notwith-
standing that some range extensions have been
reported. Lecanidae species are predominantly
(sub)tropical or warm-water, with numerous regional
and local endemics, and some Holarctic, Palaeotrop-
ical, Australasian, New World, and Old World taxa
illustrating more complex patterns.
Also Brachionidae contains taxa with well-docu-
mented ranges (see Pejler, 1977; Dumont, 1983). An
update on the distribution of some Brachionidae is as
follows:
Anuraeopsis
Of the eight species considered valid here, four are
regional endemics. Whereas A. cristata Be¯ rzin¸s
ˇ,
A. miracleae Koste and A. urawensis Sudzuki are
rare, taxonomically difficult and may have been
overlooked, the two Neotropical taxa (A. quadrian-
tennata (Koste) and A. sioli Koste are meaningful, as
they are unmistakable and have been recorded
repeatedly. As all Anuraeopsis species are warm-
water animals, and as the only reliable endemics are
Neotropical, it can be hypothesized that the taxon
may be of Neotropical origin.
Brachionus
This species-rich and predominantly warm-water
genus contains 29 endemic (sub)species, most of
which are Neotropical (9) or Australian (7). There are
only three Oriental, and one Afrotropical endemics.
Three taxa are American but probably of Neotropical
origin (B. havanaensis Rousselet, B. satanicus
Rousselet and B. zahniseri Ahlstrom). Brachionus
dichotomus reductus Koste & Shiel is Australasian
and most likely of Australian origin, by its relation
with the Australian B. dichotomus dichotomus Shep-
hard. Most of the Neotropical and Australian
endemics are phylogenetically and taxonomically
distinct. This is much less clear for the Palaearctic
and Nearctic endemics, most of which are clear
relativesof the B. plicatilis complex (B. asplanchnoides,
B. ibericus,B. spatiosus).The emerging pattern is one of
centered endemicity in South America and Australia,
with hardly any endemicity in Africa and the Northern
hemisphere. Such a pattern may hint at a late Cretaceous
South American-Antarctic-Australian (see Hay et al.,
1999), rather than a Gondwanan (Dumont, 1983)origin
of the taxon.
Fig 3 Rotifer diversity in
the major biogeographic
regions. Number of species/
number of genera. Upper:
Monogononta, Lower:
Bdelloidea.
PA—Palaearctic;
NA—Nearctic;
NT—Neotropical;
AT—Afrotropical ;
OL—Oriental;
AU—Australasia;
PAC—Pacific Oceanic
Islands; ANT—Antarctic
56 Hydrobiologia (2008) 595:49–59
123
Keratella
Within Brachionidae, Keratella is the genus with the
highest degree of endemicity (52%), and this may
even be an underestimate considering the confused
taxonomy of a number of species complexes like
Keratella cochlearis. Endemicity is high in the
Eastern Palearctic (K. mongoliana Segers & Rong,
K. sinensis Segers & Wang, K. trapezoida Zhuge &
Huang, K. wangi Zhuge & Huang and K. zhugeae
Segers & Rong) and Northern Nearctic (K. armadura
Stemberger, K. canadensis Be¯rzin¸s
ˇ,K. crassa
Ahlstrom, K. taurocephala Myers). Here, a Southern
hemisphere cold-water faunal component is repre-
sented by K. kostei Paggi, K. sancta Russell
(New Zealand, Kerguelen, Macquarie Island) and
K. reducta (Huber-Pestalozzi) (Cape region, South
Africa), amongst others. Considering the relatively
small area of southern hemisphere temperate regions,
these taxa balance the northern hemisphere tem-
perate Keratella fauna. In addition, there are some
reliable Australian (e.g., K. australis Be¯rzin¸s
ˇ),
Oriental (K. edmondsoni Ahlstrom), and warm-water
Neotropical (K. nhamundaiensis Koste) endemics, as
well as Palaeotropical (K. javana Hauer) and Holarctic
(K. hiemalis Carlin) taxa. In contrast to Brachionus,
no clear general pattern emerges in Keratella.
Another remarkable genus is Macrochaetus.It
contains 6 endemics out of 13 species, 4 of which are
Neotropical. Three of these are clearly distinct and
quite primitive in lacking the elongate dorsal spines
typical of the genus. Hence, also Macrochaetus
could be Neotropical in origin. The surmised origin
of Brachionus and Macrochaetus contrasts with
Trichocerca, in which a northern hemisphere pre-
Pleistocene origin, followed by glacial extinctions in
the (west) Palearctic, was postulated to account for an
observed lack of endemics in the tropics versus high
endemicity in northeast North America (Segers,
2003).
Clearly, and notwithstanding the unsatisfactory
nature of our knowledge of their taxonomy, rotifers
do exhibit complex and fascinating patterns of
diversity and distribution as illustrated in a number
of contributions (Green, 1972; Pejler, 1977;De
Ridder, 1981; Dumont, 1983; Segers, 1996,2003).
In summary, many species are cosmopolitan, either or
not exhibiting latitudinal variation as a result of
temperature preferences. Regional differences may
result from environmental conditions such as water
chemistry. Endemism is real and occurs at diverse
geographical scales; more complex patterns exist.
Rotifer diversity is highest in the tropics, with
endemicity centered in tropical South America and
Australia; tropical Africa including Madagascar and
the Indian subcontinent are notable for their relatively
poor rotifer fauna including few endemics. Hotspots
occur in northeast North America, Australia (proba-
bly west Australia) and, in contrast to the low
endemicity on the Indian subcontinent, Southeast
Asia. On a more local scale, Lake Baikal is most
noteworthy by its high endemicity; much less is
known of other ancient lakes. (Harring & Myers,
1928; Green, 1972; Pejler, 1977, Dumont, 1983;
Segers, 1996,2001,2003). The remarkable rotifer
diversity in northeast North America, in contrast to
the low endemicity in European waters is attributed
to the presence of glacial refugia in the region during
the Pleistocene, at least for Trichocerca (Segers,
2003).
Fenchel & Finlay (2004) postulated that small-
sized organisms (\1 mm) tend to have a cosmopol-
itan distribution as a consequence of huge absolute
population sizes. At the local scale, their diversity
exceeds that of larger organisms yet at the global
scale this relation is reversed because endemism is
largely responsible for the species richness of large-
sized taxa. A latitudinal diversity gradient is absent or
weak. Monogonont rotifers appear to comply with
this pattern: their local diversity is relatively high
compared to the total species diversity of the group,
and cosmopolitanism is important. On the other hand,
a latitudinal diversity gradient is clearly evident in
rotifers (e.g., Green, 1972). Two factors may account
for this apparent contradiction: first, the statement
that all rotifer resting stages are eminently suited for
dispersal may not be correct. Such a generalization is
contradicted by the abundance of well-documented
cases of locally endemic rotifers. Second, the
monopolizing effect of large resting propagule banks
may counteract successful colonization.
Human-related issues
Rotifer distribution and diversity is largely influenced
in two ways. The most important is that of the decline
of the water quality in freshwater ecosystems. As
Hydrobiologia (2008) 595:49–59 57
123
mentioned above, the most diverse rotifer assem-
blages can be found in soft, slightly acidic, oligo- to
mesotrophic waters. These are particularly vulnerable
to eutrophication and salinization. Regarding water
pollution by pesticides, there are numerous laboratory
studies on rotifer ecotoxicology, even using rotifers
as test organisms for ecotoxicological assessments.
The effects of pollutants on rotifer diversity in nature
also has been studied. Rotifers are often less sensitive
to insecticides than cladocerans and their sensitivity
to specific compounds varies widely. They also
exhibit indirect effects from exposure to toxicants,
e.g., through reduction of competition from more
sensitive organisms or cascading food web effects
(see Wallace et al., 2006).
Due to the large dispersal and colonization capac-
ities of many species, rotifers are easily transported to
new habitats by man. An illustrative case is that of
Filinia camasecla Myers, 1938, which was described
from the Panama Canal zone; however, the species
has subsequently never been found back in the
Americas, but has been shown to be a relatively
common Oriental species. Several additional
instances are known of rotifers being introduced to
regions where they did not naturally occur before
(e.g., Dartnall, 2005; see Wallace et al., 2006). This
phenomenon may have been going on for a long time
(see Pejler, 1977) and may be responsible for isolated
records of regionally common species outside their
natural range. It may, however, have passed unno-
ticed because of the small size of rotifers and dearth
of comprehensive studies. The same reasons explain
why rotifers have hardly been used in biodiversity
assessments and conservation, notwithstanding their
economic relevance in aquaculture.
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... Monogonont rotifers, one of the most abundant groups of aquatic invertebrates, play a pivotal role in nutrient recycling and the transfer of energy in many freshwater ecosystems (Bonecker & Aoyagui, 2005). This class of rotifers includes 1,570 named species-level taxa but the actual numbers may be much higher given the occurrence of cryptic species and the unsatisfactory nature of rotifer taxonomy (Segers, 2008). Morphology may not be sufficient to differentiate among monogonont species due to phenotypic plasticity, resulting from predator-induced defenses, producing morphological variation within a single species (Gilbert, 2001;Wallace, 2002). ...
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